Programatically download specific file types from a website

Here is a comparison between an actual image and the corresponding grayscale canvas image. You can learn about Blobs here. Blobs are objects that are used to represent raw immutable data. Blob objects store information about the type and size of data they contain, making them very useful for storing and working file contents on the browser. In fact, the File object is a special extension of the Blob interface. Blob objects can be obtained from a couple of sources:.

Here are some code samples for the aforementioned blob object sources:. It is one thing to obtain a blob object and another thing altogether to work with it. One thing you want to be able to do is to read the content of the blob. That sounds like a good opportunity to use a FileReader object. You can learn about FileReader objects here. A FileReader object provides some very helpful methods for asynchronously reading the content of blob objects or files in different ways.

The FileReader interface has pretty good browser support and supports reading blob data as follows as at the time of this writing :.

Here is what a typical object URL looks like:. The URL. Here is what it looks like:. It is important to note that, this method will always return a new object URL each time it is called, even if it is called with the same blob object. Whenever an object URL is created, it stays around for the lifetime of the document on which it was created.

Usually, the browser will release all object URLs when the document is being unloaded. However, it is important that you release object URLs whenever they are no longer needed in order to improve performance and minimize memory usage. It takes the object URL to be released as its argument. For example:.

We have also seen how we can programmatically extract or generate content in the browser using Web APIs. In this section, we will examine how we can download programmatically generate content in the browser, leveraging all we have learned from the beginning of the article and what we already know about blobs and object URLs. The logic of our helper function can be broken down as follows:. Here is what an implementation of this helper function will look like:. That was a pretty straightforward implementation of the download link helper function.

Notice that the helper triggers a one-off automatic download of the blob content whenever it is called. Also notice that the helper function takes a filename as its second argument, which is very useful for setting the default filename for the downloaded file. IOUtils; import org. RestTemplate; import com. Gson; import com. Any other JSON parser can be used here.

File ;. IOException ;. URI ;. URISyntaxException ;. URL ;. Charset ;. List ;. Map ;. FileUtils ;. IOUtils ;. RestTemplate ;. Gson ;. GsonBuilder ;. Any other REST client. Any other HTTP. This is deprecated and should not be used. Use lastModified instead. A string specifying the file's path relative to the base directory selected in a directory picker that is, a file picker in which the webkitdirectory attribute is set. This is non-standard and should be used with caution.

Often you won't want the user to be able to pick any arbitrary type of file; instead, you often want them to select files of a specific type or types. For example, if your file input lets users upload a profile picture, you probably want them to select web-compatible image formats, such as JPEG or PNG. Acceptable file types can be specified with the accept attribute, which takes a comma-separated list of allowed file extensions or MIME types.

Some examples:. It may look similar, but if you try selecting a file with this input, you'll see that the file picker only lets you select the file types specified in the accept value the exact interface differs across browsers and operating systems.

The accept attribute doesn't validate the types of the selected files; it provides hints for browsers to guide users towards selecting the correct file types. It is still possible in most cases for users to toggle an option in the file chooser that makes it possible to override this and select any file they wish, and then choose incorrect file types. Because of this, you should make sure that the accept attribute is backed up by appropriate server-side validation.

In this example, we'll present a slightly more advanced file chooser that takes advantage of the file information available in the HTMLInputElement.

Note: You can see the complete source code for this example on GitHub — file-example. Note: opacity is used to hide the file input instead of visibility: hidden or display: none , because assistive technology interprets the latter two styles to mean the file input isn't interactive. Next, we add an event listener to the input to listen for changes to its selected value changes in this case, when files are selected.

The event listener invokes our custom updateImageDisplay function. The custom validFileType function takes a File object as a parameter, then uses Array.

If a match is found, the function returns true.